Small-ish RMH with several design limitations

[giordanobernard]

Greeting to everyone,
I just recently registered in the forum but have been interested in rocket stoves and rocket mass heaters for quite a while.
I'm currently trying to design a rocket mass heater for my parents house and have even exchanged a couple of e-mails with Peter about it (you already know that but the guy is just so gentle and generous) then a lot of life happened and I never get back to the project until now. I've decided to post it here to ask for your help in trying and finalize it so it can at some point be builded.

A couple of information first:

This stove won't be the principal heating system for the hause
It's dimension is heavily limited by the space it has been reserved for it and the design my parents would like it to have.
The previous limitations themself dictate the choice of the core as a DSR in order to have an appropriate TOP GAP without having to reduce the core size too much

The project is currently in a kind of quasi-developed stage.

I highly appreciate you help.


Here all the files regarding the project can be found:
drive.google.com/drive/folders/1r1Y4m4lCSqNDt2QEY6HZjNEjPQKPSajZ?usp=sharing

[fiedia]

welcome giordanobernard,

Could you please explain how the hot gases exit the bell. it looks like the exit is on bell upper part.

[giordanobernard]

Thank you for your answer Fiedia.

I hadn't put it in the file, but I was thinking on either placing a tube as in the upgraded file or to build a masonry channel in that corner and just put a piece of cimney horizontally to make the connection to the existing chimney.

I tried to attach an image just here but couldn't do it.

[fiedia]

Looks fine. The hole in the flue is here to clean the horizontal pipe?

You will have to insulate the pipe inside the bell. You can also start with one horizontal pipe at floor level through the wall, it would avoid one flue right angle bend but I guess you are using an existing hole in the wall?

I would put the firebox as high as possible. It is really much better for loading and cleaning. The firebox window radiates also much heat. It is a pity to have this nice radiation at feet level.

I am not familiar with DSR2 core. Peterberg should validate your specific design. I guess that the afterburner chamber will be colder in your design as when it sits entirely on top of the firebox.

[peterberg]

The DSR2 could be built as a sidwinder, with the top box at right angles relative to the firebox. The bell is another matter, the half-cylindrycal dome should be done in firebricks, I'd say. And even then, there's no guarantee it won't develop cracks. Maybe with multople layers of cob with lots of straw in it. More like 50 cm long straw dipped in clay slip.

Furthermore, the entrance to the chimney is difficult. Best to build a masonry chimney piece in the bell, piercing through the dome and connecting to the opening in the wall. Be careful, the exhaust opening in this masonry column's footing should be at least 2 times the cross section area of the system size/chimney.

This is an implementation geared to the user's wishes for specific esthetics, unfortunately. The upshot of all this: there's no prior known experience using this layout so you are on your own as the proverbial lone drummer ahead of the troops.

[giordanobernard]

Thank you Peter.
I'm going to do a general review of the project and point out several critilcal points I'm not so sure how to approach.
About the advices you already gave me, I want to understand where they come from:

    1. Why does the opening on the base of the chimney column have to be 2xCSA o the chimney?
    2. Why the half cilindrical dome is subject to cracking? It is because of a too great heat gradient from the top of the half bell to the bottom of it, or because of the flames potentially extending out of the afterburn into the bell?

I hope to soon post a well detailed review of the critical parts for a common discussion about them, hopefully this will be helpful also for other people that might encounter similar problems.

[peterberg]

Nov 2, 2023 12:06:11 GMT -8 giordanobernard said:

1. Why does the opening on the base of the chimney column have to be 2xCSA o the chimney?

The vast majority, if not all, of gases are entering the exhaust opening from above and the sides. The bottom isn't important here, just the edges of the opening. This is called a stream profile, the length of this will determine how smooth the stream is rounding the corner. As a rule of thumb, for a 90 degree change of direction, you'll need 150% of the chimney CSA. For a 180 degree change, 200%. It's exactly the same mechnism that makes it necessary to have an adequate top gap.
Have a look at this: donkey32.proboards.com/thread/337/helpful-hint
The underlying principle: gases, especially fast moving gases aren't exactly willing to change direction so it's best to provide lots of room, all in order to avoid pinch points. And the reason for avoiding restrictions: the batchrocket is extremely picky about friction in the smoke path. Found out, the hard way.

Nov 2, 2023 12:06:11 GMT -8 giordanobernard said:

2. Why the half cilindrical dome is subject to cracking? It is because of a too great heat gradient from the top of the half bell to the bottom of it, or because of the flames potentially extending out of the afterburn into the bell?

Bricks that are heating up will expand. The more heat, the more expansion, causing lots of cracks. Some masonry constructions are prone to "walking bricks", due to the heating and cooling cycle they will walk apart further and further.
Your dome should be capable to withstand 500 ºC and more, so it would be best to use bricks that are sturdy enough to withstand the repeating heating/cooling cycle. Normal bricks aren't up to that, heated up to more than half their baking temperature will cause them to shrink, permanently.

[giordanobernard]

I see. It is nice to learn new things!

How is this expansion-shrinking cycle solved with a flat ceiling of the bell?

Also, if the stream profile is the critical mesure in the TOP GAP too, doesn't that mean that provided you have an enough heat resistant material, the TOP GAP can be very narrow if it is "large" enough?

I'm thinking, the exit of the AFTERBURN is a 90° degree turn but it's really narrow albeit large, why can it be so?

[peterberg]

Nov 2, 2023 13:55:04 GMT -8 giordanobernard said:

How is this expansion-shrinking cycle solved with a flat ceiling of the bell?

By keeping it mechanically separated from the walls. Two large refractory slabs on a seal of superwool is one solution, the steel T-profiles/firebricks construction of the Mallorca build is another.

Nov 2, 2023 13:55:04 GMT -8 giordanobernard said:

Also, if the stream profile is the critical mesure in the TOP GAP too, doesn't that mean that provided you have an enough heat resistant material, the TOP GAP can be very narrow if it is "large" enough?

Tricky question. There's a critical minimum top gap, a concept that's easy to grasp. That said, why would you size the top gap down to the very edge of killing the workings of the heater? Personally, I started with twice the diameter of the riser and shifted to half of that. Never tried to find out the MINIMUM specs, wanted to stay on the very safe side of things. So I can't tell you what the smallest top gap is and still "large" enough. I could imagine that while coming out of the riser the gas expansion is at its fullest so a spaciously gap would be appropiate.

Nov 2, 2023 13:55:04 GMT -8 giordanobernard said:

I'm thinking, the exit of the AFTERBURN is a 90° degree turn but it's really narrow albeit large, why can it be so?

I am assuming you mean in the DSR2 design. There's a simple answer, though. In this design there's a wider horizontal space after the short riser, which is part of the afterburner. The exit of this space is system size again. The idea behind it is a "folded" riser with a wider space halfway its trajectory. The upshot of this: this top box is there to allow the gases to expand and sending it through the narrow exit next. The net result is a combustion core that is reluctant to burn faster than a certain rate, preventing the dreaded "thermal runaway". And in one stroke, the afterburner "length" is much shorter.

As you may understand, all designs are the result of endlessly experimenting. The why and how came later, and as such I could be totally wrong about what is triggering what. The designs are tight, just do what is recommended and it works. If you are planning to do things differently, you are on your own.

[giordanobernard]

thank you again, Peter.
I'm trying to figure it out if I have the spaces to actually build a "ziggurat" top for this stove on the inside to solve many of the problems the half-Barrel brings with it and then just build the half barrel shape over that. I think I might be able to. I hope to update the skp file soon and move further into the process. 

[giordanobernard]

Finally made some progress in the design and uploading review3.skp

drive.google.com/file/d/18fbM6eG4XDGvRfqXbirswRKuMMNq2alk/view?usp=drive_link

I think I've managed to keep all reccomandations given till now and created a flat top for the stove with a faux areated concrete barrel on top (I think it's best for this part to not radiate too much heat anyway because it would mostly radiate it towards the ceiling and wood of the wall. Eventually the side facing the "middle" of the room could be filled with common bricks to allow it to radiate a bit more.)

I think we can prolong the opening in the wall to allow for a low exit from the stove but I still need advice on how to join with the existent chimney, I was thinking maybe building a chimney column right along the existing one and leaving a masonry corridor (buit to respect stream profile requisites) from the stove to that new chimney column could be a solution? it would be hard to clean such column though.

You can see that the side of the stove facing the corridor (where the firebox opening faces) is distanced from the cross laminated timber structure. Should I design for a reflective metal sheet mounted on spacers all around there? And is it safe to then fill the gap between the stove and the plasterboards\spacers with thin areated concrete slabs? I need to fill that gap somehow with something supporting the same finishing plaster of the stove.

The DSR2 core is still to be redesigned according to the materials purcheaseable in my area but its dimensions will not vary much.
Speaking of the core, I'm going to have a ISA of about 4.90 square meters, so that call for a 150 system. My existent chimney is a 180 refractory chimney, is this going to create any problem?

Thank you again for your time and advices.

[fiedia]

May be I am not calculating correctly but I get a different ISA: height (0.66m) * (2 long sides (1.1m) + 2 short sides (0.9m)) = 2.6 m2. I would not add the roof because it is insulated : 0.9*1.1 = 1m2.

I would build a taller bell and put the firebox 40 to 50cm higher. It is better if the bell and firebox radiate at body level. Be aware that this type of oven efficiency relies in radiating heat rather than ambient air heating. The bigger the radiating surface, the better you will feel.

In addition, a firebox at floor level is really not comfortable for loading and cleaning.

I would also put 2 or 3 layers of bricks on the side along the wall facing the flue. It will store more heat and insulate this side.

Why do you not build a 180mm system? It is better to over scale. I had a 125mm system and it barely heated my kitchen (30m2).

[giordanobernard]

Nov 6, 2023 6:31:21 GMT -8 fiedia said:

May be I am not calculating correctly but I get a different ISA: height (0.66m) * (2 long sides (1.1m) + 2 short sides (0.9m)) = 2.6 m2. I would not add the roof because it is insulated : 0.9*1.1 = 1m2.

I would build a taller bell and put the firebox 40 to 50cm higher. It is better if the bell and firebox radiate at body level. Be aware that this type of oven efficiency relies in radiating heat rather than ambient air heating. The bigger the radiating surface, the better you will feel.

In addition, a firebox at floor level is really not comfortable for loading and cleaning.

I would also put 2 or 3 layers of bricks on the side along the wall facing the flue. It will store more heat and insulate this side.

Why do you not build a 180mm system? It is better to over scale. I had a 125mm system and it barely heated my kitchen (30m2).

I did take into consideration every internal vertical surface and the ceiling too. You can find in the model a group called ISA which where my number comes from. I don't know if I considered surfaces I should not.

I can't build a taller bell because of the prexisting niche for inserting the stove and the design my parents want the stove to have. (for myself I'd just go with the bigger parallelepide I can fit in there)

I can't build a 180 system or higher the firebox because the limited size of the stove milits both my ISA and maximium height of the core in order to respect the top gap.
Also, wouldn't building a sistem too big for the ISA just produce unnecessarily hot gases at the exit of the stove?